Communications systems transmit or receive signals in a portion of the radio frequency spectrum known as a frequency band. Usually, communications systems are forced to operate in close proximity to other bands. From the point of view of the communications system, all other signals can be regarded as interfering signals. Hence, communications systems attempt to use the band of interest while excluding interference from all other bands.
Therefore, most communications systems usually use some form of filter to allow the use of the information in the desired frequency band while excluding all other interference. These filters are often called bandpass filters or band limiting filters, and they may be used in receivers, transmitters, or both. Indeed, band limiting filters may be useful in any electronic system which requires frequency selectivity.
There are a number of desirable characteristics for band limiting filters. Firstly, it is often desirable for band limiting filters to be narrowband, that is, they are required to minimize the undesired information outside the band of interest that is passed by the filter. Sometimes it is also desirable for band limiting filters to provide gain thereby increasing the signal level, since often the communications system either transmits from a low power signal source or receives a low power signal.
One known implementation of band limiting filters uses all passive components. Filters of this implementation are often known as passive filters. In general, passive filters are known to be implemented with combinations of lumped elements such as inductors and capacitors, or with equivalent distributed resonant structures such as ceramics, crystals, resonant transmission lines, and the like, or with hybrid lumped element structures with partially resonant structures, such as shortened transmission lines with capacitors or inductors added.
A limitation of the passive approach is that passive filters do not provide gain because there is no active element. Furthermore, passive elements will have manufacturing tolerances which, at radio frequencies, can be substantial enough to degrade the frequency performance of the filter. This degradation in frequency performance could result in a filter that is not narrowband enough or may be at the wrong frequency. Additionally, passive structures on a semiconductor substrate in an integrated circuit can be relatively large which may directly affect the cost of the system.
To achieve a signal gain as well as a narrowband response, an active filter is desirable. An active filter contains at least one active element, such as a transistor or diode, to provide a gain. Active filters may be realized as amplifiers where the frequency selectivity is achieved by the quality of the input and output impedance matching. Therefore, the amplifier may provide gain to a narrowband of the frequency spectrum while attenuating other out of band interferers.
A known type of active filter is called a super regenerative receiver. This device alternates itself from completely turned off, through an amplifying state, into an oscillating state. As the device passes through the amplifying state and close to the oscillating state, the amplifying bandwidth narrows Thus, there is a period of time, after startup, but before the gain of the device saturates and oscillation begins, where the device is capable of narrowband amplification. This operating period, or state, is desirable for band limiting filter operation.
Traditionally, there have been a number of limitations to the super regenerative receiver approach. Firstly, it has been difficult to control the operation of the device during the desirable amplification stage. Specifically, the devices have not been sufficiently controlled so that the desirable amplifier state is maintained close to oscillation. Thus, the devices were usually pulsed on and off. This pulsed mode of operation limits the selectivity of the device due to sampling alias responses, and thus makes the device vulnerable to interference. Secondly, although in the amplifying state the devices can achieve very high sensitivity, they are difficult to tune and are highly non-linear, As a result of these factors, super regenerative receivers have seen limited usefulness in general high performance communications applications.
For the foregoing reasons, there is a need to provide an amplifier for high gain, narrowband signal amplification.